Known diesel fuel components include the reaction products of Fischer-Tropsch methane condensation processes, for example the process known as Shell Middle Distillate Synthesis (van der Burgt et al, “The Shell Middle Distillate Synthesis Process”, paper delivered at the 5th Synfuels Worldwide Symposium, Washington D.C., November 1985; see also the November 1989 publication of the same title from Shell International Petroleum Company Ltd, London, UK). These Fischer-Tropsch derived gas oils are low in undesirable fuel components such as sulfur, nitrogen and aromatics and are typically blended with other diesel base fuels, for instance petroleum derived gas oils, to modify the base fuel properties.
Other known diesel fuel components include the so-called “biofuels” which derive from biological materials. Examples include alcohols such as methanol and ethanol, and vegetable oils and their derivatives. Most such biofuels are oxygenates, i.e. they contain oxygen in their structure which influences their physicochemical properties and their performance relative to that of straight hydrocarbon fuels.
Biofuels such as rapeseed methyl ester (RME) have been included in diesel fuel blends in order to reduce life cycle greenhouse gas emissions and restore lubricity in particular to fuels which have been subjected to high levels of hydrotreatment to reduce sulfur levels. They are however known to increase the density of the blend with respect to the base fuel and often to increase regulated emissions such as of nitrogen oxides (NOx).
Current commercially available compression ignition (diesel) engines tend to be optimized to run on fuels having a desired specification, in particular a density within a specified range. The blending of a standard commercial diesel base fuel with other fuel components, to modify the overall fuel properties and/or performance, can therefore have an adverse impact on the performance of the blend in the engines for which it is intended.
A further complication can arise when an engine is run on a fuel blend instead of a standard base fuel. Within the engine's fuel injection system, the fuel comes into contact with a range of elastomeric materials, in particular fuel pump seals. In use, many of these elastomers swell on contact with diesel fuel to an extent which depends on the chemistry of the fuel, aromatic fuel components and oxygenates serving for instance to promote swelling.
New elastomers in a fuel injection system tend to equilibrate with a uniform fuel diet and can thus provide with reasonable consistency the required level of sealing. They become vulnerable, however, if a change in fuel diet causes any significant change in the degree of elastomer swell. In the worst cases a mixed fuel diet can stress the elastomeric components of an engine to such an extent that fuel leakage results. By way of example, inclusion of RME in a diesel fuel blend may cause an increase in elastomer swell and in cases engine seal failure.
For the above reasons, it is desirable for any diesel fuel blend to have an overall specification as close as possible to that of the standard commercially available diesel base fuels for which engines tend to be optimized. For example it is desirable that the density of the blend be as close as possible to that of the optimal base fuel. In other words, the blend is ideally “neutral”, or as near to neutral as possible, with respect to the relevant base fuel property.
This can however be difficult to achieve because any additional fuel component is likely to alter the properties and performance of the base fuel. Moreover the properties of a blend, in particular its effect on elastomeric engine components and on emissions performance, are not always straightforward to predict from the properties of the constituent fuels alone, the constituents often contributing in a non-linear fashion to the overall blend properties. The greater the number of fuel components in a blend, the less predictable its overall properties become.